1. Technical Field
The present invention relates to a fixing device.
2. Related Art
A typical electrophotographic image forming apparatus includes a fixing device for fixing a toner image on a recording medium such as a transfer sheet. In the fixing device, the recording medium passes through a nip portion formed by, for example, a heated rotating fixing member (e.g., a fixing roller or belt) and a rotating pressure member (e.g., a pressure roller) pressed against the fixing member. Thereby, toner of the toner image carried on the recording medium is fused, and the toner image is fixed on the recording medium.
The pressure roller usually has an outer circumferential layer made of a material having at least a certain level of frictional resistance and elasticity, such as rubber or resin. If the outer circumferential layer is made of a material that expands and contracts with temperature, the outer diameter of the pressure roller changes. This change in outer diameter is particularly prominent in a pressure roller made with an elastic layer of increased thickness, as is commonly done to reduce crumpling and misregistration of the recording medium on which the toner image is to be fixed when the recording medium is comprised of a plurality of superimposed sheets such as envelopes, for example.
Further, if recording media of different sizes pass through the pressure roller, it is difficult to maintain a uniform temperature distribution in the direction of the rotation axis of the pressure roller. A difference in temperature occurs between a recording medium passing region and recording medium non-passing regions of the pressure roller, causing a difference in thermal expansion therebetween, which results in distortion of the pressure roller.
The fixing device may be configured to suppress an increase in temperature of the recording medium non-passing regions due to the passage of many small-sized recording media through the pressure roller, and thereby prevent hot offset (i.e., adhesion of excessively fussed toner to components of the fixing device) and deterioration of the pressure roller, for example.
That is, the fixing device may be configured to include a heating roller, a pressure roller pressed against the heating roller to thermally fix a toner image on a recording medium passed through between the rollers, and a cooling device that cools the recording medium non-passing regions of the pressure roller, through which the small-sized recording media do not pass. Specifically, the cooling unit may be, for example, a blower capable of controlling the amount of air to be blown onto the recording medium non-passing regions.
However, if the pressure roller including the above-described thick elastic layer, storing a large amount of heat, and having an uneven temperature distribution is cooled with a constant amount of air blown onto the pressure roller, the change in outer diameter of the pressure roller due to the change in temperature varies in the rotation axial direction, making it difficult to maintain the shape of the pressure roller. Particularly if the pressure roller has a constricted shape including opposed end portions and a central potion smaller in diameter than the opposed end portions, it is difficult to maintain the difference between the maximum diameter of the pressure roller at the opposed end portions and the minimum diameter of the pressure roller at the central portion.